1. Field of the Invention
The invention relates to the treatment of wastes generated during the production of high purity silane and silicon. More particularly, it relates to an improved waste treatment system obviating the need for disposal of such wastes.
2. Description of the Prior Art
A process for high purity silicon production presently under development includes (1) hydrogenation of metallurgical grade silicon with hydrogen and silicon tetrachloride, (2) disproportionation of the resulting chlorosilanes to produce high purity silane, and (3) decomposition of the silane to produce high purity silicon, with recycle of by-product and waste materials. A simplified waste disposal technique proposed for incorporation in this process involves condensing a minor portion of unreacted silicon tetrachloride from the chlorosilane gas stream removed from the hydrogenation reaction. Carryover metallurgical silicon powder, metal halides and other impurities present in the gas stream separate with the condensed silicon tetrachloride. Such materials can be subjected to hydrolysis during waste disposal. The process also generates waste gases, such as hydrogen and nitrogen streams removed as dissolved gases from intermediate process liquids and during purging of the process equipment. Additionally, the waste disposal facilities for the process must be capable of handling any plant upsets and associated startup and shutdown conditions as they occur. Such requirements necessitate that the waste disposal facilities be capable of handling the complete spectrum of components present in the overall process. Such components include silicon tetrachloride, mono-, di, and trichlorosilane and silane as well as the other materials indicated above. While the simplified waste disposal technique is an advantageous one, the required capability and flexibility for the handling of such a complete spectrum of gaseous and liquid wastes imposes severe restrictions on the waste removal system. In addition, the waste materials, including hydrated silica sludge, recovered from the process must be disposed of in an environmentally acceptable manner.
Earlier efforts to develop a suitable waste disposal process focused on conventional practices in operating plants producing a range of silicon products. In one such operation involving various silicon-containing wastes including chlorosilanes and silicon tetrachloride, the waste disposal practice includes the burning of the gas waste with subsequent scrubbing of the resultant gas to remove particulate matter. Hydrolysis and lime neutralization are employed in the treatment of liquid waste. Such a waste disposal process is acceptable, but it generates considerable hydrated silica sludge. As in the newer approach referred to above, this sludge must be either stored in settling ponds or processed through filters for ultimate removal as landfill or the like. In addition, operating difficulties, including equipment plugging, necessitate frequent shutdowns for equipment cleaning, thereby increasing the overall costs of the operation intended for the production of low-cost, high purity silicon.
Another prior art practice is based on relatively small operations for the production of cylinder lots of high purity silane from a trichlorosilane feedstock. Waste disposal involves the combustion of the waste gases that are normally chlorosilane or silane itself and the scrubbing of the combusted gas mixture with a series arrangement of high energy Venturi scrubbers. The scrubbed gas is then ejected to the atmosphere, whereas liquid waste passes to a settling tank for eventual disposal to either sewer or landfill. This arrangement is again acceptable for the particular operation, but is based on a relatively narrow range of wastes compared with those expected from a facility intended to produce high purity silane and silicon from metallurgical grade material. This prior art arrangement, as noted, is adapted to handle relatively low levels of waste from a small production operation as compared to the magnitude of wastes to be treated in a commercially feasible metallurgical silicon to high purity silicon production facility.
Based on such prior art practices, it has been proposed for the desired silicon production to combine the burning of gaseous wastes with the scrubbing of the resulting vent gases and subsequent neutralization of liquid waste by lime and water addition thereto. The sludge thus formed would be filtered for removal as landfill, whereas the clear liquid would be used in the scrubbing step. Such an approach, however, has the same limitations and drawbacks as pertain to the prior art practices, accelerated by the broad range of waste components encountered in the desired silicon production by hydrogenation of metallurgical silicon to trichlorosiliane, disproportionation of the trichlorosilane to silane or mono or dichlorosilane, and decomposition of said silane, mono or dichlorosilane to high purity silicon in a fluid bed or free space reaction zone or by a Siemens-type reaction.
There is a desire in the art therefore, for improvements in the waste treatment or disposal operations pertaining to the conversion of metallurgical grade silicon to high purity silane or silicon. There is, in particular, a need to provide for such waste treatment in an environmentally acceptable manner that does not unduly add an incremental cost to the high purity silane or silicon operation. This is of importance in ligh of the genuine need in the art for the development of features to enhance the prospects for effectively utilizing high purity silicon on a commercially feasible basis for solor cell and semiconductor applications.
It is an object of the invention, therefore, to provide an improved waste treatment system for use in the production of high purity silane and silicon from metallurgical grade silicon.
It is another object of the invention to provide a waste treatment process obviating the waste disposal requirements of conventional techniques used in the production of silane and organo-silicon products.
It is a further object of the invention to provide an improved waste treatment approach enhancing the prospects for producing low-cost, high purity silane and silicon from metallurgical grade silicon.
With these and other objects in mind, the invention is hereinafter disclosed in detail, the novel features thereof being particularly pointed out in the appended claims.